Champions League semifinals, leg 1: predictions and key battles

The Champions League returns and with just four teams left, the path to glory is wide open. Any of the remaining sides has a chance but equally, question marks surrounding their challenge. Can Liverpool crack Lionel Messi & Co. to deny their Treble push?

Here's everything you need to know, from the superheroes and the sidekicks who can decide the ties to the keys for victory.

Barcelona vs. Liverpool

BACKSTORY: Barcelona won La Liga on Saturday and will play Valencia in the Spanish Cup final later this month, which means they're still on track for the Treble. It would be their third since 2008, which is some achievement when you consider that it had only been achieved three times in the first 40 years of European football.

Liverpool reached the Champions League final last year, of course, but unlike then -- when they went out of domestic contention early -- they are now neck-and-neck with Manchester City for the Premier League title. They've won 10 games in a row in all competitions and look stronger and more complete than last year.

HOW THEY MATCH UP: It used to be a rarity for a team to score 100 goals in all competitions, but Liverpool passed that mark on Saturday and Barcelona did it back in February. Yet though both can (and do) score plenty, there's a more patient, defensive dimension to what they do as well. Jurgen Klopp only wheels out his "heavy metal" all-out press intermittently, and compared to prior incarnations, this Barca side are more safety-oriented in the middle of the park. That fact that both have outstanding goalkeepers in Marc-Andre ter Stegen and Alisson adds another layer of solidity.

BARCELONA'S STAR TO WATCH: Lionel Messi. He's averaging more than a goal a game (again) while playing an even bigger role than before in terms of dropping deep and creating chances for others. You can find your own superlatives here.

Can Lionel Messi deliver on his preseason promise of Champions League glory?

LIVERPOOL'S STAR TO WATCH:LIVERPOOL'S STAR TO WATCH:Virgil van Dijk. Few signings have been as transformative in recent years as the Dutchman's arrival at Anfield in January 2018. A colossus in the air, his work in reading the game and marshalling a back four in which both fullbacks are often attacking at the same time is second to none.

X-FACTOR, BARCELONA:Jordi Alba. He has 16 assists in all competitions this year from left back and a nearly supernatural understanding with Messi, who often cuts in from the opposite flank. He'll be up against Trent Alexander-Arnold and Mohamed Salah, which means Liverpool will need to get a midfielder helping out when he advances. Except they can't overcommit to him, because Messi is on the opposite side...

X-FACTOR, LIVERPOOL:Roberto Firmino. He's a center-forward who doesn't score that much (he has broken the 20-goal mark just twice in his career) but his prodigious movement and off-the-ball work rate more than make up for it. Between Sergio Busquets in midfield and Gerard Pique at the back, Barcelona won't want to lose sight of him.

BARCELONA NEED TO... Dictate the game in midfield and pin back Alexander-Arnold and Andrew Robertson, Liverpool's full-backs, while denying service to Salah and Sadio Mane, particularly in transition.

LIVERPOOL NEED TO... Be aware of Messi at all times, but particularly when he drops back to create. Ensure the front three don't get isolated, while also maintaining their shape.

PREDICTION: Liverpool 1-2 Barcelona. Two evenly matched sides but one of them has Messi, the other does not.